An abbrev is a word which has been defined to expand into
a specified expansion. When you insert a word-separator character
following the abbrev, that expands the abbrev--replacing the abbrev
with its expansion. For example, if foo is defined as an abbrev
expanding to find outer otter, then you can insert find
outer otter. into the buffer by typing f o o ..

Abbrevs expand only when Abbrev mode (a minor mode) is enabled.
Disabling Abbrev mode does not cause abbrev definitions to be forgotten,
but they do not expand until Abbrev mode is enabled again. The command
M-x abbrev-mode toggles Abbrev mode; with a numeric argument, it
turns Abbrev mode on if the argument is positive, off otherwise.
Section 32.1. abbrev-mode is also a variable; Abbrev mode is
on when the variable is non-nil. The variable abbrev-mode
automatically becomes local to the current buffer when it is set.

Abbrev definitions can be mode-specific--active only in one major
mode. Abbrevs can also have global definitions that are active in
all major modes. The same abbrev can have a global definition and various
mode-specific definitions for different major modes. A mode-specific
definition for the current major mode overrides a global definition.

Abbrevs can be defined interactively during the editing session. Lists
of abbrev definitions can also be saved in files and reloaded in later
sessions. Some users keep extensive lists of abbrevs that they load in
every session.